Hi Shan,
I don't think any environmental regulation will allow you to have a disssolved gas in your discharge water to the environmental, especially I will assumed that the gas that is still dissolved and may flash is a hydrocarbon gas.
By having this,you are actually creating a HC gas cloud which may accumulated and cause disaster.
My suggestion is to operate the water skimmer at atmospheric pressure and flash the gas even before the mixed liquid is going to the skimmer (i.e to have like a gas boot or degasser upstream of last stage water treatment vessel and make the fluid flow from the gas boot or degassser by gravity) and from the last stage water treatment vessel, to pump the water for discharge.